1. Technical Field
This disclosure relates to an image forming apparatus, an image processing method, and a printer driver, and more particularly to an image forming apparatus, an image processing method, and a printer driver that reproduce black by using a color ink besides black ink or by mixing black ink and a color ink besides black ink.
2. Description of the Related Art
As for known image forming apparatuses, there are printers, facsimile machines, copiers, and complex apparatuses thereof. One example of such image forming apparatuses is an inkjet recording apparatus which is able record high definition images at high speed by jetting ink from an ink recording head to a recording medium (a medium on which ink (recording liquid) is applied including, for example, paper, transfer paper, OHP sheet).
The inkjet recording apparatus, at first, grew to widespread spread use owing to its inexpensiveness and ability to generate high quality images by using a predetermined paper. Currently, however, electrophotographic type laser printers, which are employed in offices, are employed as recording apparatuses for outputting color images.
Typically, the ink used for the inkjet recording apparatus contains water as a main component, a color agent, and a wetting agent (glycerin) for preventing clogging. The color agent includes dyes and pigments. A dye type ink is often employed for coloring color areas from the aspect of satisfactory color property and stability.
Images obtained by the dye type ink are inferior compared to those obtained by a pigment type ink from the aspect of fastness property (e.g. light fastness, water resistance). Although water resistance of the images obtained by the dye type ink may be improved to some extent by employing an inkjet dedicated recording paper which includes an ink absorbing layer, satisfactory results cannot be attained by printing on plain paper.
In order to alleviate such problems which occur in recording with dye type ink on plain paper, the use of pigment type ink including organic pigment, carbon black, etc., is proposed or put into practical use. Since pigment does not have the same water solubility as dye, the pigment is normally mixed with a dispersant, is subject to a dispersion process in water, and is provided as an aqueous ink upon reaching a stable dispersed state.
Although improved light fastness and water resistance can be attained by using the pigment, it is difficult to obtain satisfactory image density, color property, and color reproducibility, etc., especially in a case of performing high speed printing on plain paper. Furthermore, the use of the pigment has problems such as character bleeding, color boundary bleeding, and unsatisfactory performance in double side printing and ink drying property (fixing property).
One example for solving the problems occurring in a case of printing with pigment type ink on plain paper is shown in Japanese Laid-Open Patent Application No.6-171072. In this example, an ink containing a pigment, a polymer dispersant, and a resin emulsion is employed, in which the amount of solid matter to be adhered to a corresponding area on a recording paper is adjusted under a condition of 100% duty printing. This example reduces creation of uneven print areas due to cohesion of pigment regardless of the type of paper. Furthermore, this example provides an image having high print density without print character bleeding.
Another example for solving the problems occurring in a case of printing with pigment type ink on plain paper is shown in Japanese Laid-Open Patent Application No.2000-355159. In this example, an ink containing a surface processed pigment and a penetrating agent as its components is employed, in which the amount of the ink to be jetted to a corresponding area in a recording medium is adjusted. The use of this ink enables reduction of irregular bleeding, fast drying of ink on the recording medium, attaining of a high print density and satisfactory print image.
The first example disclosed in Japanese Laid-Open Patent Application No.6-171072 is able to improve print density and reduce character bleeding since the ink in this example provides a considerably high contact angle equal to or greater than 70 degrees with respect to a predetermined paper such as plain paper. However, in a case of performing 100% duty printing on a recording paper, the necessary amount of solid material to be adhered to a corresponding area is approximately several tens of ng/m2. This is undesirable from the aspect of ink fixing property (drying property). More particularly, in a case of repetitively printing a large amount of paper at high speed, stains are created between the sheets of paper. Therefore, this example is unsuitable for high speed printing. Furthermore, in performing 100% duty printing, white streaks may be created at plain parts and character parts of the paper due to the high contact angle. Furthermore, owing to the high contact angle, color bleeding easily occurs between adjacent printed dots in color boundary portions of the recording paper.
The second example disclosed in Japanese Laid-Open Patent Application No.2000-355159 is advantageous from the aspect of ink drying property (fixing property) since it employs a penetrating agent. Furthermore, this example is suited for high speed printing since no stains are created in a case of printing a large amount of paper at high speed. However, since the ink in this example contains a penetrating agent, character bleeding is caused in a case of printing on plain paper. More particularly, since the ink penetrates the plain paper in a depthwise direction, this example is unsuitable for double side printing on plain paper.
As another example for solving the problems in using pigment type ink for printing on plain paper, there is an ink containing at least a pigment, a water soluble organic solvent, a polyol with a carbon number of 8 or more or glycol ether, and water. This example proposes to provide (1) satisfactory color tone (sufficient coloring property, color reproducibility), (2) high image density, (3) clear images without feathering or color bleeding of characters and images, (4) sufficient endurance for performing double side printing without ink penetrating through to the back of the paper, (5) high drying property suitable for high speed printing, and (6) satisfactory fastness property (e.g. light fastness, water resistance).
Although laser printers, conventionally, were predominantly used for printing on plain paper, inkjet printers are currently able to print likewise high quality images on plain paper as a result of experiments and research with the pigment type ink. Therefore, inkjet printers are widely used in offices owing to their low cost and ease of maintenance.
There is, however, still a demand for printing high quality images on paper dedicated for inkjet printing. In addition to the widespread use of digital scanners, high quality digital cameras are widely used. Therefore, common users are able to manipulate data of high quality images and use the dedicated paper (particularly, glossy paper providing a glossiness on the paper surface) for outputting images with a quality corresponding to that of the input data.
However, the pigment type ink developed for improving image quality for printing on plain paper is not necessarily suitable for printing on glossy paper. That is, although the glossy paper has applied a coating for absorbing ink and providing glossiness to the paper surface, pigment type ink tends to cause pigment components to accumulate on the surface of the coating. The accumulation of the pigment components causes the recording surface to lose its glossiness and also causes dried up pigment components to peel from the paper surface.
In order to solve this problem, there is one widely used method of employing a dye type ink as a color ink and a pigment type ink as a black ink. There is also another method of providing a pigment type ink adjusted for glossy paper and a pigment type ink adjusted for plain paper. The latter method, however, uses a large amount of space for mounting a head unit or an ink cartridge, and is therefore difficult to employ for an apparatus used for commonly used paper (e.g. A4 size paper).
There is also a method of employing an intermediate ink which is adjusted to be applicable for both glossy and plain paper. This method, however, demonstrates a half measure image quality which is unable to provide sufficient ink performance either for glossy paper or for plain paper. There is also a method of employing a glossy paper having its glossiness controlled for making loss of glossiness difficult to recognize in a case where a pigment type ink is used. This method, however, has lower image quality compared to a case of employing a combination of dye type ink and glossy paper.
Meanwhile, the method of using a dye type ink as a color ink and a pigment type ink as a black ink is employed considering that most text images are recorded on plain paper with black ink. The use of a pigment type ink suited for recording on plain paper as a black ink improves the image quality of the text image (plain paper image).
Furthermore, since a black color (composite black) can be reproduced by combining cyan (C), magenta (M), and yellow (Y), a composite black ink, being a combination of inks of the three colors, can be used as black ink for recording on glossy paper. Therefore, a satisfactory fixablity on a recording paper can be attained without loss of glossiness.
The method of employing composite black ink for reproducing black, however, has a problem in adjusting gray balance (for the sake of convenience, black represented in gradation levels is referred to as gray). Since colors are reproduced by subtractive color mixing, black is, theoretically, reproducible by combining C, M, and Y. However, since the inks for each color do not necessarily have ideal C, M, Y color characteristics, the inks are subject to a process of, for example, a γ correction process or a halftone process in order to reproduce the ideal C, M, Y color characteristics.
More particularly, in the halftone process, various modifications are made for improving not only toning but also granularity and texture. For example, in a case of inkjet recording apparatuses, a so-called error diffusion method is widely employed in which the spatial frequency characteristic of dot arrangements is of a high frequency. Furthermore, a dither method (as shown in Japanese Laid-Open Patent Application No.11-150652) serves to improve visual granularity by providing a blue noise characteristic to the halftoned dot arrangement for each color.
Regarding each gradation level of the resultant black composite, these methods have extreme difficulty in coordinating the resultant black composites to an ideal gray(s). As shown in FIG. 20, these methods result to variably distributed hues. In the example shown in FIG. 20, the tinting of gray in each gradation level changes from a cyan-like gray to a blue-like gray, and then to a magenta-like gray.
Such disarrangement in gray balance leads to a considerable reduction of image quality. Although a method expressing gradation with continuous tone change is not used for expressing typical photograph images, it is often used for expressing artificial gradation images including three dimensional CAD images, computer graphic images, and presentation images. In outputting the latter images, a variable gray balance and changing hue leads to unevenness of color and results in reduction of image quality.
Reduction of image quality due to unevenness of color becomes more apparent in a case of using composite black as an alternative for reproducing black on glossy paper, for example, since monochrome images and black characters (black letters) are processed with use of composite black.